Method for determining the equivalent fracture permeability of a fracture network in a subsurface multi-layered medium

Abstract

Method for determining the equivalent fracture permeability of a fracture network in a subsurface multi-layered medium from a known representation of this network. The equivalent fracture permeability of a fractured network in a subsurface multi-layered medium, is determined by discretizing with a specific procedure each fracture (F) of the fracture network in fracture elements (R) (such as rectangles for example) and defining nodes N representing interconnected fracture elements in each layer of the medium and determining fluid flows (steady-state flows e.g.) through the discretized network while imposing boundary pressure conditions and fluid transmissivities to each couple of neighboring nodes. The method allows for a systematic linking of fractured reservoir characterization models with dual-porosity simulators in order to create a more realistic modeling of a fractured subsurface geological structure. The method can be implemented for example in oil production by reservoir engineers for obtaining reliable flow predictions.

We claim: 
     
       1. Method for determining the equivalent fracture permeability of a fractured network in a subsurface fractured multi-layered medium, from a predetermined representation of said network, comprising the steps of discretizing each fracture (F) of the fracture network in fracture elements and defining nodes (N) representing interconnected fracture elements in each layer of the medium; and   determining fluid flows through the fracture network while imposing boundary pressure conditions, and fluid transmissivities to each couple of neighboring nodes.   
     
     
       2. A method according to claim 1, comprising the steps of: partitioning the medium in a set of parallel layers (L) each extending in a reference plane (Ox, Oy) perpendicular to a reference axis (Oz) and defined each by a co-ordinate (zO) along said axis;   partitioning each fracture in a series of rectangles (R) limited along said reference axis by two adjacent layers (L) and itemizing the rectangles by associating therewith geometrical and physical attributes;   positioning nodes (N) in each layer for all the interconnected fractures (F); and   for all the couples of neighboring nodes, calculating transmissivity factors and solving flow equations to determine the equivalent permeabilities of the medium in three orthogonal directions.   
     
     
       3. A method according to claim 1, wherein the equivalent permeability of the medium includes directly determining an equivalent permeability tensor and calibrating absolute values of permeability from well tests results. 
     
     
       4. A method according to claim 2 wherein said geometrical and physical attributes are co-ordinates and sizes of the rectangles and hydraulic conductivities of the fractures.